Polymeric materials have many advantages in both commercial and industrial applications compared to traditional materials such as glass. Typically they afford their user design freedom and advantageous properties such as decreased weight and reduced cost of production. An interesting area of application for the surface engineering of polymeric substrates is the design of ultrathin coatings for use in ‘everyday’ environments, such as easy-clean coatings on smart phones to mirrors and decorative (colourful) coatings in the automotive industry. Prolonged exposure to these ‘everyday’ environmental conditions requires such ultrathin coatings to possess a level of robustness against factors such as acidic or caustic environments, temperature variations from as low as −80° C. to highs of +80° C., variations in relative humidity, as well as abrasive conditions inflicted by everyday wear and tear.
Additionally, in almost all applications employing such coatings, there is a need for the coating to maintain its integrity over extended periods of operation under varying environmental conditions. That is, the coating should not significantly change reflectivity or colour within this time, nor should it delaminate or break away from the underlying substrate.
However, the application of appropriate coatings to polymeric materials is particularly difficult given that traditional coating methods generally require substrates that can withstand high temperatures (often >150° C.). As most common polymeric materials have relatively low softening temperatures, there is a paucity of appropriate coatings for these substrates.
To date the majority of work on the development of robust abrasion-resistant ultrathin coatings has thus focussed on the deposition of hard compounds such as borides, carbides and nitrides to hard/heat-resistant substrates such as metal and ceramics. However, the high temperatures used in the deposition techniques are inappropriate for use on polymeric substrates. Therefore, there is a need for ultrathin coatings that can be applied to polymeric substrates with relatively low softening temperatures, which are still capable of delivering desired physical properties such as abrasion resistance, reflectivity (preferably with a R % greater than 50%) and neutral (or a desired) colour.
With specific regard to colour, reference throughout this specification to a “neutral” colour is reference to a colour that is defined by measured L*, a* and b* values in accordance with the 1976 CIE L*a*b* Space (or CIELAB) colour model, which is an approximately uniform colour scale organised in cube form. In the orthogonal a* and b* colour axes, positive a* values are red, negative a* values are green, positive b* values are yellow and negative b* values are blue, while the vertical scale for lightness (or greyscale) L* runs from 0 (black) to 100 (white), allowing the positioning of a total colour E in three points. The Chroma (C*) of the colour is defined as V(a*+2+b*+2), and is used to quantify the magnitude of the colour independent of its lightness. Ideally, for the colour E to be neutral, the C* value will be less than or equal to 1 and the colour E will thus be close to the neutral L* axis.
The above discussion of background is included to explain the context of the present invention. It is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge at the priority date of any one of the claims.